Sacher cake

This is the ORIGINAL Sacher torte as sold at Hotel Sacher.
The cake is pretty dense and dry, it's layered and the cover is hard: it's not ganache. It's actually made by making simple sugar syrup and melting the chocolate in the syrup.
The cake is made without any rising agents, except the egg whites. This Mary Berry gets right.
The apricot preserve is to be warmed, sieved and have a little dark rum added. It works to preserve the cake.
A Sacher can be preserved for two weeks after finishing, because of the apricots and chocolate cover.

The Demel Sacher is baked according to the recipe of the original creator, and it isn't layered.

Mary Berry's Sacher is not a Sacher.
Delia Smith's Sacher is absolutely not a Sacher!

Sacher Cake (Sachertorte)
Viennese Cooking, O. & A. Hess, adapted for American use [Crown Publishing:New York] 1952 (p. 229)

This is the original recipe, obtained through the courtesy of Mrs. Anna Sacher.
 
3/4 cup butter
6 1/2 oz. semi-sweet chocolate
3/4 cup sugar
8 egg yolks
1 cup flour
10 egg whites, stiffly beaten

2 tbls. apricot jam

icing:
1 cup sugar
1/3 cup water
7 oz. semi-sweet chocolate

Beat butter until creamy. Melt chocolate. Add sugar and chocolate to butter; stir. Add egg yolks one at a time. Add flour. Fold in egg whites.
Grease and butter 8-9" cake tin. Pour mixture in.
Bake in 275 degree F. oven about 1 hour. Test with toothpick or straw.
Remove to board; cool.

Cut top off and turn bottom up.
Heat apricot jam slightly and spread over top.

Cover with chocolate icing, prepared as follows:
Cook sugar and water to thin thread.
Melt chocolate in top of double boiler.
Add sugar gradually to chocolate.
Stir constantly until icing coats the spoon.
Pour on top of cake

Note: If desired, split cake into 2 or 3 layers. Fill with apricot jam or whipped cream.